In a statement released after President Bush's address to the nation, Homeland Security Secretary Tom Ridge said intelligence officials believe terrorists will attempt multiple attacks against U.S. and coalition targets worldwide in the event of a U.S.-led military campaign against Iraq.

The statement said reports from a wide range of intelligence sources -- some of them highly reliable -- suggest the al Qaeda network probably would claim it is acting on behalf of the Muslim world and Iraqi people rather than the regime of Saddam Hussein.

In a conference call Monday evening between CIA officials and the nation's governors, officials said the intelligence community believes there is a near certainty of attacks that would involve mass casualties with maximum economic, psychological and symbolic impact.

Intelligence officials believe al Qaeda is in the final stages of planning a large-scale attack overseas, officials said in the conference call. The CIA said it believes al Qaeda will rationalize the use of chemical, biological or radiological weapons based on the large number of casualties the United States may inflict in its military campaign.

There was particular concern expressed about buildings, subways, and enclosed areas, the CIA said. There was also concern about Iraqi sleeper agents proficient in assassination, kidnapping, and bombing.

"Orange" is the second-highest level in the nation's five-tiered, color-coded threat alert system. The nation was last put on that level February 7 because of increased "chatter" among suspected terrorist cells and the proximity to the Muslim Hajj.

No incidents occurred, and the level was lowered to "yellow" nearly three weeks later.

Plan to detain asylum applicants

Ridge also announced the start of a national plan called Operation Liberty Shield to enhance security nationwide.

The plan includes detaining asylum applicants from nations where al Qaeda members and sympathizers and other terrorist groups are known to have operated.

Calling it a "reasonable and prudent temporary action," the Homeland Security Department said the applicants would be detained while they are being processed so U.S. officials can stay in contact with them while deciding on their asylum claims.

Border protection and increased security at airports and railways, and greater road security are also parts of the plan. Homeland Security called for temporary flight restrictions over certain U.S. cities, including Washington and New York, though the Federal Aviation Administration told CNN it had not yet issued those rules.

New airport measures

Because of the higher threat level, the Transportation Security Administration, which is in charge of security at the nation's airports, announced four new measures Monday night.

The TSA is ordering airports to conduct random inspections of vehicles, increase canine patrols, and increase the overall law enforcement presence in and around airports.

The agency will also be putting up temporary signs inside airports asking the public to be aware of the increased threat level and to report unattended bags and suspicious behavior.

The Coast Guard will protect offshore petroleum sites near large coastal population centers, and security is being increased at chemical facilities, nuclear power plants, key electrical grids, bridges and subways.

Operation Liberty Shield will also increase monitoring of diseases and food security, Homeland Security said. The Agriculture Department is alerting food producers to step up their security -- such as inspecting all vehicles and escorting all visitors -- and is helping monitor feedlots, stockyards, and import and storage areas.

Imported food will also come under increased scrutiny by the Department of Health and Human Services, which is asking health departments and medical care providers to report unusual diseases or disease patterns.

"In the meantime, as on the other occasions when the national threat level has been increased, we encourage members of the public to continue their daily work, family and leisure activities with a heightened awareness," Ridge said.